Packing composition for removal of zinc from zinc-coated articles



Patented Dec. 27, 1949 Harry A. Ecliman, Chicago, 111., assignor toCrane 00., Chicago, 111., a corporation of Illinois No Drawing.Application May 18, 1946, Serial No. 670,831

. 2 Claims. 1

This invention relates to a heat treatment process and more particularlyis concerned with the object for removing zinc as a coating from zinccoated materials.

The invention is specially concerned with a method for accomplishingsuch object and avoiding all the undesirable effects resultant from theuse of treatments heretofore employed.

A further important object is to provide for a method for removing zincas a coating by the employment of a simple combined packing andtreatment method using containers.

A further object is to eliminate the undesirable effect of zinc fumesexperienced in previous methods employed in which the zinc fumes werefound objectionable.

In order to obtain a better appreciation of the significance of thiscontribution it should be realized at the outset that zinc coatedproducts, particularly those which are hot galvanized and machined arefrequently found unsuitable for commercial use because of the subsequentdiscovery of a defective or non-uniform coating. It should be furtherunderstood that such articles however, can be used later as a non-coatedproduct if the zinc coating can be satisfactorily removed from thesurface of such article, as for instance, the removal of the zincgenerally has been done by dissolving the zinc coating in an acidsolution. However, this procedure has been costly and the surfaces ofthe material treated often are objectionably attacked by the acid,realizing that the acid in many instances penetrates into fissures orcracks and pin-holes frequently of very small size on the surface andthen later resulting in surface corrosion.

One of the more important advantages obtained by the use of thisinvention is the elimination of all of the undesirable effects of theacid treatment of material.

Primarily it consists of heating the zinc coated articles, such as pipefittings from which the coat is to be removed, to a temperature not toexceed approximately 1700 F. and affecting oxidation of the zinc withoutmaterially or appreciably changing the physical properties of thearticle itself. Preferably it has been found that the heating requiredshould be done in containers, such as pots or the like.

It is important to realize however, that unless special care is takenduring the course of the heat treatment, the zinc fumes formed willattack the uncoated areas, such as machine sur-' faces, threads and thelike, and after subsequent oxidation, it results in a zinc oxide depositadhering to such surfaces which is of course objectionable particularlyin the case of parts finished to a specific machine tolerance, as forexample pipe threads or other finished surfaces in which the matter of arelatively close fit is important.

It has been discovered that by packing the zinc coated articles to betreated in a suitable metallic material of desired fineness the usualdetrimental effect of the volatilizing zinc in the way of zinc fumes iseliminated. It has been observed that the zinc fumes formed react withthe surface of the metallic particles constituting the packed materialand thereby forming alloys of zinc; thus the metallic packing materialpresent in this manner functions as a fixing agent for the volatilizingzinc and the alloys of zinc thus formed are later broken down byoxidation.

It will be apparent to those skilled in the art that zinc coatingsobtained by hot dipping consist of an outer layer of substantially purezinc and an inner layer which is relatively thin and is alloyed to theiron surfaces. It will be further understood that the zinc in the outerlayer of such coating volatilizes readily when heated at relatively lowtemperatures, while the alloy layer beneath is more stable and iseliminated upon subsequent oxidation.

It has been found that zinc coated products such as pipe fittings forexample, treated in accordance with the procedure hereinafter describedin greater detail shows substantially no zinc present on the surfaceeven when tested with a chemical reagent such as a copper sulphatesolution; and a further and more important benefit obtained is shown inthe fact that machined or uncoated surfaces remain clean and unaffected.The latter is a very significant benefit derived.

As to the packing material employed, it may be any metal or metallicalloy having a melting point exceeding the temperature used in thecourse of heating and which is capable of reacting with the volatilizingzinc. Ferrous materials, however, have been found most desirable. Atypical mixture found acceptable is one consisting of twentyfive percent(25%) to seventy-five percent (75%) iron particles as essentialcomponents, carbonates up to ten percent and an amount of solidcarbonaceous material not to exceed ten percent (10%) and the balanceconsisting of an in ert refractory such as sand or clay.

It is preferable that the principal constituents of the packing materialconstitutes finely divided particles thus securing a large surface areafor the reaction with volatilizing zinc fumes; the particle sizehowever, should at the same time be large enough to allow free passageof gases and fumes. Comparatively-coarse materials, such as cast ironchips, however, have been used-successfully. Other readily availableferrous materials, such as grinding dust have alsoproven satisfactory.

For the best performance it has "been found 'desirable to mix themetallic packing with a certain amount of inert or refractory substance,such as clay, kaolin, or sand to prevent the metal particles fromsticking together after the heat treatment. More specifically coarsesand has beeniound mostsuitableand it may be present vin .amounts ,up to75% of the compounded mixture. i

It should be understood that the removal of the "zinc coveringdependsupon the oxidation of the zinc by carbon 'dioxide (CO2) To insure this.reaction it has been found desirable to add a certain amount ofcarbonaceous matter, such as powderedseacoal or'charcoal, to themixture. The carbon present provides for a certain reducingatmosphere'consisting principally of carbon monoxide and it will .bereadily apparent to those 'skilled in the art that carbon monoxide at atemperature exceeding approximately 1150 -F.

an increase in temperature, the carburizing action of carbon monoxidebecomes increasingly rapid. It:is .desirableiunder this condition thatthe metallic packing component in themixture should respondtocarburization and this reaction .is facilitated by the large surface.of the relatively finely divided metallic particles in the .-mixture.

Small :amounts of zinc present on the surface of these particles do notaffect the carburization. Metallic carbonates present in the-mixturefurther exert :a catalyzing effect energizing the carburizing reaction..As-a result of the carburizing actionof carbon monoxide, carbon dioxideis evidenced by the following formula:

3Fe+2CO=Fe3C'-l-CO2 The carbon dioxide'thus resulting serves as anoxidizing agent for the'zinc as illustrated by the following reaction:

Zn+COz=ZnO+CO .Itwill be readily-:apparentthat the abovereactions'mentionedare vitally important in .eifecting the oxidationofthe zinc. .abovetwo'reactions into one, the process can be Bysummarizing the expressed as below:

' Zn-l-1 3Fe+CO=ZnO+Fe3C In order to facilitate the oxidation of thezinc it has been found desirable to incorporate-a-cer- -tainamountbfasuitable-carbonate in the packing mixture and the amount ofcarbonate added should preferably be in proportion to the carbon contentof the packing compound so that the atmosphere within the packingmixture is maintained reducing. Too high carbon dioxide content willexert undesirable oxidizing eiiect of the ferrous material treated.Carbonates used in the mixture may be any carbonate suitable to generatecarbon dioxide during the process of heating, such as calcium carbonate,magnesium carbonate, sodium carbonate, barium carbonate and the like..It has :also been found that mixtures of the carbonatemay be used toadvantage depending upon the processing temperatures used. Satisfactoryresults have also been obtained without the additionof-ca-rbonate to themixture. Under this condition, however, the procedure is relativelyslower and less eflicient.

'As'the-carburizing action of carbon monoxide is rapidly increased withthe corresponding increase .in temperature insofar as the heating isconcerned, the oxidation of the zinc similarly proceeds proportionatelyfaster as the higher "temperatures are attained. Metallic carbonatespresent facilitate further the carburizing reac- 'tion.

Material treated, such as cast iron, is limited to a relativelyzlow'maximumprocessing temperature in order to preserve :the properties ofthe iron. However, :malleable 'iron may be processed atLtemperatureslin-the vicinity .of 1600 F. without any appreciable changein :its property and this has been confirmed :by :tabulated comparativephysical properties obtained on test bars poured from the same .-iron.

Insignificant amounts of zinc may be detected by chemical anal-ysison-the-outer surface of the processed articles and the ;-amounts are sosmall as to be truly insignificant-and-can be regarded asimpunityinsofar as the effect upon the metal surface is concerned.

.Insummarizing the rbenefitsof this process, it has been found possibletosalvage hotgalvanized pipe ,fittingsior example, heretofore notsuitable for commercial use due to thedefective zinc coating but byremoving the zinc coating these fittings have been salvagedandsubsequently sold as black fittings, so termed by the trade.

.Itishould of course'be quite apparent that the detailedanalysis withrespect to the various elements above recited is only average orillustrative and it may vary accordingly. Therefore the scope of theinvention should'be measured by the following .claims in relation to theprior art, as herein appended.

I claim:

'1. A packing materiaLsuitable .for the removal of the element zinc frommachined zinc-coated articles whereby thelatter are treated by heatingsaid articlespredeterminedly while packed in said packing material, thepacking consisting of 25 to iron par.ticles as essential components,carbonates upto 10%, an amount 'of solid carbonaelement zinc "frommachined zinc-coated articles by heating said articles'while packedin'said packing material to atemperature low enough not to exert-anydetrimental effect on the articles but high enough' to "e'ffect desiredremoval of the zinc, the said packing material consisting of ironparticles-mot toexceed 75% as=essential-' components,

carbonates up to 10%, an=amount of solid car- 5 bonaceous fuel materialapproximating 10% and Number the balance an inert refractory materialsuch as 360,732 sand and clay. 499,018 HARRY A. ECKMAN. 702,764 51,061,447 REFERENCES CITED 1,202,886 The following references are ofrecord in the 2,287,227 file of this patent:

UNITED STATES PATENTS 10 Number Number Name Date 1 ,4 2

66,310 Dodds July 2, 1867 Name Date Sawyer Apr. 5, 1887 Frey June 6,1893 Babe et a1. June 17, 1902 Burkey May 13, 1913 Phelps Oct. 31, 1916Bowsher June 23, 1942 FOREIGN PATENTS Country Date Great Britain Sept.13, 1889

